1. Field of the Invention
The present invention relates to an apparatus and a method for dressing a polishing pad for use in a wafer polishing apparatus.
2. Description of the Related Art
A rotary wafer polishing apparatus for polishing surfaces of semiconductor wafers is generally constructed as follows. A wafer polishing head for holding a wafer and a platen including a polishing pad affixed to the platen are arranged in opposed relation. The polishing pad is rotated while the surface of the wafer is pressed against the polishing pad and a slurry containing polishing abrasive grains is supplied. Simultaneously, the wafer polishing head is rotated to perform planetary or oscillatory motion on the polishing pad, whereby the wafer is polished.
It is known in the above wafer polishing apparatus that repeating the process of polishing a wafer reduces roughness of the polishing pad and gradually deteriorates the polishing performance. For this reason, each time the wafer polishing process is finished, the polishing pad is dressed by using polishing-pad dressing apparatus as shown in FIG. 7.
Referring to FIG. 7, the dressing apparatus comprises a dresser 11 affixed to a dresser support 10, and an arm 12 for supporting the dresser 11 rotatably in the circumferential direction with a dressing surface 11a facing downward. A ball joint (not shown) is provided at a joint portion between the arm 12 and the dresser support 10 so that the dressing surface 11a of the dresser 11 is held parallel to a polishing pad 14 regardless of an angle at which the arm 12 inclines. Also, the dresser 11 is movable in the vertical and horizontal directions toward or away from the polishing pad 14 affixed to a platen 13 with operation of an arm elevating/lowering mechanism and an arm advancing/retracting mechanism (both not shown) which are coupled to a base end of the arm 12.
In the above dressing apparatus, by rotating the polishing pad 14 while the dresser 11 is pressed against the polishing pad 14, frictional force generated between the dresser 11 and the polishing pad 14 causes the dresser 11 to rotate in the same direction as the polishing pad 14, and brings the dresser 11 into slide contact with the surface of the polishing pad 14. As a result, the surface of the polishing pad 14 is dressed.
However, a conventional dressing apparatus has such a tendency that the amount of dressing (i.e., the amount by which the polishing pad is dressed) in each step of dressing increases gradually, though by small degrees, toward the center of the polishing pad. In other words, each time the polishing pad is subjected to dressing, the surface of the polishing pad is cut in the form conically recessed toward the center, and an extent of excessive cutting on the inner side increases gradually.
The reason is thought to reside in that a dresser is arranged to locate entirely within an area covered by the radius R of the polishing pad when viewed from above vertically.
On condition that the contact pressure of the dresser is uniform, the amount of dressing of the polishing pad is determined depending on a sliding length of dresser abrasive grains at radial positions of the polishing pad, as seen from the following formula (I): EQU z(x)=k.multidot.L(x).multidot.P(x) (I)
z(x): amount of dressing at position spaced distance x from the center of the polishing pad PA1 k: proportional coefficient PA1 L(x): sliding length of abrasive grains at position spaced distance x from the center of the polishing pad PA1 P(x): contact pressure of the dresser at position spaced distance x from the center of the polishing pad
Further, the sliding length L(x) is determined depending on the rotational speed of the dresser, the surface configuration of the dresser abrasive grains, and the density of the dresser abrasive grains. Usually, the sliding length L(x) tends to have a larger value in an area closer to the center of the polishing pad.
In the conventional dressing apparatus, because the dresser is arranged to locate entirely within an area covered by the radius R of the polishing pad, the contact pressure of the dresser against the polishing pad is substantially uniform. As seen from the formula (1), therefore, the amount of dressing is increased on the inner peripheral side of the polishing pad in which the sliding length of the abrasive grains is comparatively long, thus resulting in a phenomenon of inner-side excessive cutting. Accordingly, repeating the dressing increases gradually a minus gradient of the surface configuration of the polishing pad toward the center from the outer periphery thereof, and hence increases the gradient of the pad surface in the conically recessed form.
This eventually raises a problem of impairing uniformity in each wafer polishing process.
Such a variation of uniformity in each wafer polishing process has been hitherto regarded to be so small as falling in sufficiently allowable range, but a demand for higher uniformity has become keen in recent years.
At present, the above problem is dealt with, for example, by frequently replacing the polishing pad with a fresh one. This solution however gives rise to the secondary problems that an operation cost is pushed up and the operation must be suspended for a while for replacement of the polishing pad.
Moreover, in the conventional dressing apparatus, because the dressing surface of a dresser is clogged with pad chippings cut by the dresser and slurry particles during the dressing of a polishing pad, the polishing pad is unevenly dressed. Accordingly, the machining accuracy of a wafer polished by the polishing pad deteriorates.
At present, therefore, the above problem is dealt with by cleaning the dresser after being used for the dressing, for example, with a method of spraying pure water to the dressing surface of the dresser or keeping the dresser in water to rinse the dressing surface. However, the problem of clogging of the dresser occurred during the dressing cannot be overcome at the current state of the art.